ES3012158T3 - Separator and secondary battery including the same - Google Patents

Abstract

The present invention relates to a separator and a secondary battery comprising the same, the separator comprising a porous substrate and a conductive layer disposed on the porous substrate, wherein the conductive layer comprises a carbon nanotube structure in which a plurality of single-walled carbon nanotube units are coupled to each other side by side, and the carbon nanotube structure has an average particle diameter of 2-500 nm. (Automatic translation with Google Translate, no legal value)

Description

DESCRIPTION

Separator and secondary battery that includes the same

Technical field

Cross-reference to related applications

This application claims priority to Korean Patent Application No. 10-2020-0112867, filed on September 4, 2020.

Technical field

The present invention relates to a separator including a porous substrate and a conductive layer disposed on the porous substrate, wherein the conductive layer includes a carbon nanotube structure in which a plurality of single-walled carbon nanotube units are bonded together side by side, and the carbon nanotube structure has an average diameter of 2 nm to 500 nm, and to a secondary battery including the same.

Background of the technique

The demand for batteries as a power source has increased significantly as technological development and demand for mobile devices have recently increased, and a variety of research has consequently been conducted on batteries capable of meeting diverse needs. Particularly as a power source for such devices, research has been actively conducted on lithium secondary batteries that offer excellent cycle and life characteristics, as well as high energy density.

A lithium secondary battery refers to a battery comprising an electrode assembly comprising a positive electrode comprising a positive electrode active material capable of intercalating/deintercalating lithium ions, a negative electrode comprising a negative electrode active material capable of intercalating/deintercalating lithium ions, and a separator disposed between the positive electrode and the negative electrode; and an electrolyte containing lithium ions.

Since electrons must move easily between the positive and negative electrodes (hereinafter referred to as "electrode"), a conductive electrode path must be ensured. To this end, the electrode uses a current collector having a layer of active material disposed on one surface thereof, and the active material layer includes a conductive agent. Furthermore, in order to improve the conductive electrode path, techniques have been introduced to modify the surface of the current collector or to uniformly disperse the conductive agent. However, improving the electrode conductive path using only a conventional method using the current collector or the conductive agent is insufficient.

Therefore, in the present specification, a separator capable of improving the conductive path of the electrode and a secondary battery including the same are introduced.

Document JP 6439293 B2 discloses separators.

Disclosure of the invention

Technical problem

An aspect of the present invention provides a separator capable of improving the input/output characteristics of a battery by improving the conductive path of an electrode while maintaining the degree of lithium ion diffusion of the separator.

Another aspect of the present invention provides a secondary battery including the separator.

Technical solution

According to one aspect of the present invention, there is provided a separator including a porous substrate and a conductive layer disposed on the porous substrate, wherein the conductive layer includes a carbon nanotube structure in which a plurality of single-walled carbon nanotube units are bonded together side by side, and the carbon nanotube structure has an average diameter of 2 nm to 500 nm.

According to another aspect of the present invention, a secondary battery is provided including the separator.

Advantageous effects

Since a separator according to the present invention can improve the conductive path of an electrode in contact with a conductive layer by including the conductive layer including a carbon nanotube structure in which a plurality of single-walled carbon nanotube units are bonded together side by side, the separator can improve the input/output characteristics of a battery by reducing the resistance of the battery. Furthermore, since the conductive layer has little effect on the pore structure of a porous substrate (or porous substrate and inorganic coating layer) in the separator, it can minimize a decrease in the degree of diffusion of lithium ions in the separator while improving the conductive path.

Brief description of the drawings

Figure 1 is a scanning electron microscope (SEM) image of the separator of Comparative Example 1; Figure 2 is a SEM image of the separator of Example 1;

Figure 3 is SEM images of the separator in example 2;

Figure 4 is an SEM image of the separator of comparative example 3;

Figure 5 is a transmission electron microscope (TEM) image of the carbon nanotube structure of Preparation Example 1;

Figure 6 is a TEM image of the single-walled carbon nanotube unit of Preparation Example 2;

Figure 7 is a schematic view of a battery according to an embodiment of the present invention;

Figure 8 is a schematic view of a portion of a battery according to an embodiment of the present invention; Figure 9 is a schematic view of a portion of a battery according to an embodiment of the present invention; Figure 10 is a schematic view of a portion of a battery according to an embodiment of the present invention; and

Figure 11 is a schematic view of a portion of a battery according to an embodiment of the present invention.

Mode for carrying out the invention

It is to be understood that the expressions or terms used in the specification and claims are not to be construed as having the meaning defined in commonly used dictionaries, and it is further to be understood that the expressions or terms should be construed as having a meaning that is consistent with their meaning in the context of the relevant art and the technical idea of the invention, based on the principle that an inventor may appropriately define the meaning of the expressions or terms to better explain the invention. The terms used in the present specification are used to merely describe exemplary embodiments, but are not intended to limit the invention. Terms in a singular form may include plural forms unless otherwise referenced.

It shall further be understood that the terms “include,” “comprise,” or “have” in this specification specify the presence of indicated features, numbers, steps, elements, or combinations thereof, but do not exclude the presence or addition of one or more other features, numbers, steps, elements, or combinations thereof.

In the present specification, the term “specific surface area” is measured by a Brunauer-Emmett-Teller (BET) method, in which, specifically, the specific surface area can be calculated from the adsorption amount of nitrogen gas at the temperature of liquid nitrogen (77 K) using a BELSORP-mini II device of Bel Japan Inc.

The term “average particle diameter (D<50>)” in the present specification may be defined as a particle diameter at a cumulative volume of 50% on a particle size distribution curve. The average particle diameter (D<50>), for example, may be measured using a laser diffraction method. The laser diffraction method can generally measure a particle diameter ranging from a submicron level to a few mm and can obtain results with high repeatability and high resolution.

In the present invention, the term “single-walled carbon nanotube unit” indicates a unit in the form of a single-walled tube composed of carbon atoms, and the term “multi-walled carbon nanotube unit” indicates a unit in the form of a multi-walled tube composed of carbon atoms.

Hereinafter, the present invention will be described in detail.

Separator

A separator according to the present invention includes a porous substrate and a conductive layer disposed on the porous substrate, wherein the conductive layer includes a carbon nanotube structure in which a plurality of single-walled carbon nanotube units are bonded together side by side, and the carbon nanotube structure may have an average diameter of 2 nm to 500 nm.

1) Porous substrate

The porous substrate may be a porous polymeric film substrate or a porous polymeric nonwoven substrate.

The porous polymer film substrate may be a porous polymer film formed by polyolefin such as polyethylene, polypropylene, polybutylene, and polypentene, and such a polyolefin porous polymer film substrate, for example, exhibits a quenching function at a temperature of 80°C to 130°C.

In this case, the porous polyolefin polymer film may be formed by any one of polyolefin-based polymers such as polyethylene, such as high-density polyethylene, linear low-density polyethylene, low-density polyethylene, and ultra-high molecular weight polyethylene, polypropylene, polybutylene, and polypentene, or may be formed by a polymer obtained by blending two or more thereof.

Furthermore, the porous polymer film substrate can be prepared by forming it into a film shape using various polymers, such as polyester, in addition to polyolefin. Furthermore, the porous polymer film substrate can be formed into a structure in which two or more film layers are stacked, and each film layer can be formed from a single polymer, such as the polyolefin described above or polyester, or a polymer obtained by blending two or more thereof.

Furthermore, the porous polymer film substrate and the porous nonwoven substrate may be formed from any one of polyethylene terephthalate, polybutylene terephthalate, polyester, polyacetal, polyamide, polycarbonate, polyimide, polyetheretherketone, polyethersulfone, polyphenylene oxide, polyphenylene sulfide, and polyethylene-naphthalene, in addition to the above-described polyolefin, or a polymer obtained by blending these materials.

The thickness of the porous substrate is not particularly limited, but is specifically in a range of 1 |im to 100 |im, for example, 5 |im to 50 |im, and the diameter and porosity of the pores present in the porous substrate are also not particularly limited, but may be in a range of 0.01 |im to 50 |im and 10% to 95%, respectively.

2) Conductive layer

The conductive layer may be disposed on the porous substrate. Specifically, the conductive layer may be disposed on one surface of the porous substrate, and more specifically, the conductive layer may be disposed only on one surface of the porous substrate.

The conductive layer may include a carbon nanotube structure, and specifically, the conductive layer may be formed by the carbon nanotube structure.

The carbon nanotube structure may include a plurality of single-walled carbon nanotube units. Specifically, the carbon nanotube structure may be a carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are bonded together side by side. More specifically, taking into account the durability of the conductive layer and the conductive network of an electrode, the carbon nanotube structure may be a carbon nanotube structure in which 2 to 4,500, preferably 50 to 4,500, and more preferably 1,000 to 4,500 single-walled carbon nanotube units are bonded together. Considering the dispersibility of the carbon nanotube structure and the durability of the conductive layer, the carbon nanotube structure can be one in which 1,000 to 4,500 single-walled carbon nanotube units are arranged side by side and bonded together.

Single-walled carbon nanotube units can be arranged side by side and bonded together in a carbon nanotube structure (a flexible, cylindrical bundle-like structure in which the units are bonded so that their long axes are parallel to each other) to form a carbon nanotube structure. The carbon nanotube structures are interconnected in the conductive layer to form a network structure.

The structure of carbon nanotubes is different from that of a typical carbon nanotube. Specifically, a typical carbon nanotube refers to a carbon nanotube unit formed by dispersing bundle-type or entangled carbon nanotubes (a form in which single-walled carbon nanotube units or multi-walled carbon nanotube units are bonded or entangled with each other) in a dispersion medium as much as possible (a conducting agent dispersion). In this case, since the carbon nanotubes are completely dispersed in the typical conducting agent dispersion, the carbon nanotubes are present as a conducting agent dispersion in which single-stranded carbon nanotube units are dispersed. With respect to the typical conducting agent dispersion, since the carbon nanotube units are easily cut by an excessive dispersion process, the carbon nanotube units have shorter lengths than their respective initial lengths. In this case, if the separator is coated using the conductive agent dispersion, since the carbon nanotube units with a small average diameter are located in the pores in the separator, the pore volume on a surface and inside the separator is excessively reduced, and therefore, the diffusion of lithium ions into the separator does not easily occur. Furthermore, there is a problem that the carbon nanotube units are connected to each other through the pores in the separator to cause a local short circuit between a positive electrode and a negative electrode that are present with the separator disposed between them. Moreover, with respect to multi-walled carbon nanotube units, structural defects are high due to a mechanism in which nodes grow (the units are not smooth or linear, but nodes are present due to defects generated during the growth process). Therefore, multi-walled carbon nanotube units are more easily cut in the dispersion process, and short-cut multi-walled carbon nanotube units can be easily aggregated by n-n stacking driven by the unit carbon. Consequently, it is more difficult for the multi-walled carbon nanotube units to disperse more uniformly in the separator's conductive layer.

Alternatively, with respect to the carbon nanotube structure included in the conductive layer of the separator of the present invention, the carbon nanotube structure is in the form of a string in which a plurality of single-walled carbon nanotube units that maintain a relatively high crystallinity without structural defects are arranged side by side and bonded together. Since the carbon nanotube structure has a large average diameter, it is not easily arranged inside the pores of the porous substrate (or porous substrate and inorganic coating layer) of the separator, and therefore, the pore structure can be maintained. Accordingly, since the decrease in lithium ion diffusion in the separator can be minimized, the input/output characteristics of a battery can be maintained. Furthermore, since the carbon nanotube structure forms a network structure in the conductive layer, the conductive layer can act as a kind of current collector with respect to the electrode in contact with the conductive layer due to the network structure. Specifically, since the conductive layer can be in contact not only with the electrode, but also with a current-applying portion (e.g., the bottom can in a button cell, the electrode tab in a typical full cell) connected to the electrode, current can flow along the conductive layer in addition to the current collector on the electrode when current is applied from the outside. Therefore, since the current can flow uniformly across the entire electrode surface along the conductive layer, the battery resistance can be significantly improved, and the input/output characteristics and life characteristics of the battery can be improved. In particular, due to the long length and large diameter of the carbon nanotube structure, since the conductive network by the carbon nanotube structure can be formed relatively uniformly and strongly, the above-described effect of improving the conductive path is better than that of using a typical carbon nanotube unit.

In the carbon nanotube structure, the single-walled carbon nanotube unit can have an average diameter of 0.5 nm to 10 nm, for example, 1 nm to 9 nm. When the average diameter is satisfied, the effect of improving the conductive path at the electrode through the conductive layer can be maximized even by using a small amount of the carbon nanotube structure. The average diameter corresponds to an average value of diameters of the first 100 single-walled carbon nanotube units with a larger diameter and the last 100 single-walled carbon nanotube units with a smaller diameter when the surface of the prepared separator is observed by a transmission electron microscope (TEM).

In the carbon nanotube structure, the single-walled carbon nanotube unit may have an average length of 1 ^m to 100 |im, for example, from 5 |im to 50 |im. When the average length is satisfied, since a conductive path can be formed on the electrode through the conductive layer and a singular network structure can be formed in the conductive layer, the conductivity at the electrode can be maximized even with a very small amount of the carbon nanotube structure. The average length corresponds to an average value of lengths of the first 100 single-walled carbon nanotube units with a larger length and the last 100 single-walled carbon nanotube units with a smaller length when the surface of the prepared separator is observed by a TEM.

The single-walled carbon nanotube unit can have a specific surface area of 500 m2/g to 1,000 m2/g, for example, 600 m2/g to 800 m2/g. When the specific surface area satisfies the above range, since the conductive path at the electrode can be smoothly ensured by the large specific surface area, the conductivity at the electrode can be maximized even with a very small amount of the carbon nanotube structure. The specific surface area of the single-walled carbon nanotube unit can be specifically calculated from the adsorption amount of nitrogen gas at the temperature of liquid nitrogen (77 K) using a BELSORP-mini II device of Bel Japan Inc.

The carbon nanotube structure may have an average diameter of 2 nm to 500 nm, particularly 10 nm to 500 nm, and more particularly 100 nm to 500 nm. In a case where the average diameter is less than 2 nm, the carbon nanotube structure is easily broken during the dispersion process so that the conductive network cannot be easily formed, the carbon nanotube structure may block the pore structure of the porous substrate (or porous substrate and inorganic coating layer) to degrade the performance of the battery, and an internal short circuit may occur because a conductive network may be formed in a path through the separator. In a case where the average diameter is greater than 500 nm, the pore structure of the surface of the porous substrate (or porous substrate and inorganic coating layer) may be clogged due to the excessively large diameter to degrade the performance of the battery, and since the number of chains forming the conductive network is small when the same amount of the carbon nanotube structure is used, the desired effect cannot be easily achieved. The average diameter of the carbon nanotube structure corresponds to an average value of diameters of the first 100 carbon nanotube structures with larger diameters and the last 100 carbon nanotube structures with smaller diameters when observing the surface of the prepared separator by a scanning electron microscope (SEM). The carbon nanotube structure may have an average length of 1 μm to 500 μm, particularly 5 μm to 300 μm, and more particularly 10 μm to 100 μm. When the average length satisfies the above range, since the conductive network is effectively formed and the current in the electrode can be transmitted uniformly, the battery resistance can be reduced and the overall battery performance such as the input/output characteristics and battery life characteristics can be improved. The average length of the carbon nanotube structure corresponds to an average value of lengths of the first 100 carbon nanotube structures with larger lengths and lengths of the last 100 carbon nanotube structures with smaller lengths when the surface of the prepared separator is observed by SEM.

In some cases, the single-walled carbon nanotube unit can be surface treated through an oxidation treatment or nitriding treatment to improve the affinity with a dispersant.

The carbon nanotube structure may be included in an amount of 0.01 parts by weight to 20 parts by weight based on 100 parts by weight of the porous substrate, and may specifically be included in an amount of 0.01 parts by weight to 10 parts by weight. When the amount of the carbon nanotube structure satisfies the above range, since the pore structure of the porous substrate may be exposed to the surface, the surface of the separator may have a porous structure. In addition, since the conductive layer may act as a kind of current collector on the electrode surface in contact with the separator, the resistance of the battery may be reduced and the input/output characteristics of the battery may be improved.

Since the carbon nanotube structure is a highly crystalline carbon-based material, it has high temperature stability and chemical resistance. Therefore, even if the temperature in the battery is rapidly increased, the carbon nanotube structure can maintain a solid network structure. The conductive layer can have a thickness of 10 nm to 2,000 nm, particularly 50 nm to 2,000 nm, and more particularly 100 nm to 1,000 nm. When the thickness of the conductive layer satisfies the above range, since the pore structure of the porous substrate can be exposed to the surface, the surface of the separator can have a porous structure. Furthermore, since the conductive layer can act as a kind of current collector on the electrode surface in contact with the separator, the battery resistance can be reduced and the input/output characteristics of the battery can be improved. Specifically, in order to achieve the effect described above while maintaining a high battery energy density, the thickness of the conductive layer is most preferably within a range of 100 nm to 1,000 nm. The thickness can be measured by examining a cross-section of the separator using a SEM.

The conductive layer may have a surface resistance of 5*10'1 Q/a to 5*104 Q/a, for example, 5*10 Q/a to 5*104 Q/a. Since the carbon nanotube structure is used, the above range can be derived by forming a long and robust conductive network in the form of a rope and high electrical conductivity of the carbon nanotube structure. When the surface resistance of the conductive layer satisfies the above range, since the conductive layer can act as a kind of current collector on the electrode surface in contact with the separator while the surface of the separator maintains a porous structure, the resistance of the battery can be reduced and the input/output characteristics of the battery can be improved.

The conductive layer may further include an additive that covers at least a portion of the surface of the carbon nanotube structure. The additive may play a role in properly dispersing the bundle-like or tangled carbon nanotubes in the formation of the carbon nanotube structure. Because the additive is present while covering at least a portion of the surface of the carbon nanotube structure, the conductive layer cannot easily exfoliate from the separator and can be strongly bonded to the porous substrate.

The additives may be at least one selected from the group consisting of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer (PVdF-co-HFP), polyvinyl alcohol, polyacrylonitrile, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, hydrogenated nitrile-butadiene rubber, polystyrene, pyrenebutyric acid, pyrenesulfonic acid, tannic acid, pyrenemethylamine, sodium dodecyl sulfate, carboxymethylcellulose, styrene-butadiene rubber, and fluorinated rubber, and may specifically be a carboxymethylcellulose.

3) Inorganic coating layer

In some cases, the separator may also include an inorganic coating layer. The inorganic coating layer may be disposed between the porous substrate and the conductive layer. In this case, even if ignition/heat generation occurs or the temperature rises rapidly for various reasons in the battery, since the inorganic coating layer maintains the shape of the separator to prevent a short circuit between the positive and negative electrodes, the inorganic coating layer can improve battery safety.

The inorganic coating layer may include inorganic particles. The inorganic particle is not particularly limited as long as it does not reduce the conductivity and does not cause any oxidation and/or reduction reaction, that is, an electrochemical reaction, with a positive electrode collector or negative electrode in an operating voltage range (e.g., 0 V - 5 V based on Li/Li+) of the battery, and, for example, may be at least one inorganic material selected from the group consisting of AhO<3>, BaTiO<3>, CaO, CeO<2>, NiO, MgO, SiO<2>, SnO<2>, SrTiO3, TiO<2>, Y<2>O<3>, ZnO, ZrO<2>, Pb(Zr,Ti)O3 (PZT), Pb1-xLaxZP|.yTiyO3 (PLZT), Pb(Mg-,/3Nb2/3)O3-PbTiO<3>(PMN-PT), and hafnia (WO<2>).

The inorganic coating layer may further include a binder. The binder plays a role in increasing the bonding strength between the inorganic particles. The binder may include, but is not limited to, poly(vinylidene fluoride)-co-trichloroethylene, poly(methyl methacrylate), poly(ethylhexyl acrylate), poly(butyl acrylate), polyacrylonitrile, polyvinylpyrrolidone, poly(vinyl acetate), poly(ethylene-co-vinyl acetate), poly(ethylene oxide), polyarylate, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethyl pullulan, cyanoethylpoly(vinyl alcohol), cyanoethylcellulose, cyanoethylsucrose, pullulan, and carboxylmethylcellulose.

Since the carbon nanotube structure is a highly crystalline carbon-based material, it offers high temperature stability and chemical resistance. Therefore, even if the temperature in the battery increases rapidly, the carbon nanotube structure can maintain a solid network structure. Furthermore, in a case where the inorganic coating layer and the carbon nanotube structure are bonded, since the inorganic coating layer and the carbon nanotube structure maintain the shape of the separator to prevent short-circuiting between the positive and negative electrodes even if ignition/heat generation occurs or the temperature rises rapidly due to various reasons in the battery, the safety of the battery can be further improved.

The air permeability of the separator may be in a range of 50 s/100 cm3 to 500 s/100 cm3, particularly 100 s/100 cm3 to 300 s/100 cm3, and more particularly 150 s/100 cm3 to 250 s/100 cm3. The term “air permeability” means the time required for 100 cm3 of air to penetrate through the separator. Satisfying the air permeability means that the pore structure of the separator is effectively established, and, in other words, it means that the pore structure of the separator can be maintained even if the conductive layer has been formed. When the air permeability range of the separator is satisfied, since the diffusion of lithium ions can be facilitated, the inlet/outlet characteristics of the battery can be improved.

Separator preparation method

Next, a preparation method of the separator of the present invention will be described.

The preparation method of the separator of the present invention includes the steps of: preparing a carbon nanotube structure; and arranging the carbon nanotube structure on a porous substrate, wherein the carbon nanotube structure includes a structure in which a plurality of single-walled carbon nanotube units are bonded together side by side, and the carbon nanotube structure may have an average diameter of 2 nm to 500 nm. The porous substrate and the carbon nanotube structure may be the same as the porous substrate and the carbon nanotube structure described in the separator of the embodiment described above.

1) Preparation of carbon nanotube structure

The preparation of the carbon nanotube structure may include the steps of: preparing a mixed solution including a dispersion medium, a dispersant, and bundle-type single-walled carbon nanotubes (aggregate or bonded body of single-walled carbon nanotube units) (S1-1); and forming a carbon nanotube structure, in which a plurality of single-walled carbon nanotube units are bonded side by side, dispersing the bundle-type single-walled carbon nanotubes by applying a shear force to the mixed solution (S1-2).

In step S1-1, the mixed solution may be prepared by adding bundle-type single-walled carbon nanotubes and a dispersant to a dispersion medium. The bundle-type single-walled carbon nanotubes are present in the form of a bundle in which the above-described single-walled carbon nanotube units are bonded, wherein the bundle-type carbon nanotube typically includes 2 or more, substantially 500 or more, for example, 5,000 or more single-walled carbon nanotube units.

The bundle-type single-walled carbon nanotube can have a specific surface area of 500 m2/g to 1,000 m2/g, for example, 600 m2/g to 800 m2/g. When the specific surface area satisfies the above range, since the conductive path at the electrode can be smoothly guaranteed by the large specific surface area, the conductivity at the electrode can be maximized even with a very small amount of the conductive agent.

The bundle-type single-walled carbon nanotubes may be included in an amount of 0.1% by weight to 1.0% by weight, for example, 0.2% by weight to 0.5% by weight, in the mixed solution. When the amount of the bundle-type single-walled carbon nanotubes satisfies the above range, since the bundle-type single-walled carbon nanotubes are dispersed at an appropriate level, a carbon nanotube structure can be formed at an appropriate level and the dispersion stability can be improved.

The dispersion medium may include, for example, water, a polar amide-based organic solvent such as dimethylformamide (DMF), diethylformamide, dimethylacetamide (DMAc), and N-methylpyrrolidone (NMP); alcohols such as methanol, ethanol, 1-propanol, 2-propanol (isopropyl alcohol), 1-butanol (n-butanol), 2-methyl-1-propanol (isobutanol), 2-butanol (sec-butanol), 1-methyl-2-propanol (tert-butanol), pentanol, hexanol, heptanol, or octanol; glycols such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, 1,3-propanediol, 1,3-butanediol, 1,5-pentanediol, or hexylene glycol; polyhydric alcohols such as glycerin, trimethylolpropane, pentaerythritol, or sorbitol; glycol ethers such as ethylene glycol monomethyl ether, diethylene glycol monomethyl ether, triethylene glycol monomethyl ether, tetraethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monoethyl ether, triethylene glycol monoethyl ether, tetraethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, or tetraethylene glycol monobutyl ether; ketones such as acetone, methyl ethyl ketone, methyl propyl ketone, or cyclopentanone; and esters such as ethyl acetate, y-butyrolactone, and g-propiolactone, and any one of them or a mixture of two or more of them may be used, but the present invention is not limited thereto. Specifically, the dispersion medium may be water.

The dispersant may include at least one selected from hydrogenated nitrile butadiene rubber, polyvinylidene fluoride, and carboxymethylcellulose, and may specifically be carboxymethylcellulose. The dispersant may correspond to the additive of the embodiment described above.

In the conductive agent dispersion, the weight ratio of the bundle-type carbon nanotubes to the dispersant may be in a range of 1:0.1 to 1:10, for example, 1:1 to 1:10. In a case where the weight ratio satisfies the above range, since the bundle-type single-walled carbon nanotubes are dispersed at an appropriate level, a carbon nanotube structure at an appropriate level can be formed and the dispersion stability can be improved.

The solids content in the mixed solution may be in a range of 0.1% by weight to 20% by weight, for example, 1% by weight to 10% by weight. In a case where the solids content satisfies the above range, since the bundle-type single-walled carbon nanotubes are dispersed at an appropriate level, a carbon nanotube structure can be formed at an appropriate level and the dispersion stability can be improved. In addition, an electrode suspension can have a viscosity and elasticity suitable for an electrode preparation process, and also contribute to increasing the solids content of the electrode suspension.

In step S1-2, a process of dispersing the beam-type carbon nanotubes in the mixed solution can be performed using a mixing device such as a homogenizer, a bead mill, a ball mill, a basket mill, an attrition mill, a universal stirrer, a ClearMixer, a pin mill, a TK mixer, or a sonication equipment. Among them, a bead milling method is preferred because it can precisely control the diameter of the carbon nanotube structure, can achieve uniform distribution of the carbon nanotube structure, and can have cost advantages.

The bead milling method can be as follows: The mixed solution can be placed in a bead-containing container, and the container can be rotated to disperse the bundle-type single-walled carbon nanotubes.

In this case, the conditions under which the pearl grinding method is carried out are as follows.

The beads may have an average diameter of 0.5 mm to 1.5 mm, for example, 0.5 mm to 1.0 mm. In a case where the average diameter satisfies the above range, the diameter of the carbon nanotube structure can be appropriately controlled without breaking the carbon nanotube structure during the dispersion process, and a dispersion solution with a uniform composition can be prepared.

The rotation speed of the vessel may be in a range of 500 rpm to 10,000 rpm, for example, 2,000 rpm to 6,000 rpm. If the rotation speed is within the above range, the diameter of the carbon nanotube structure can be appropriately controlled without breaking the carbon nanotube structure during the dispersion process, and a dispersion solution with a uniform composition can be prepared.

The time during which bead milling is performed may be in a range of 0.5 hours to 2 hours, particularly 0.5 hours to 1.5 hours, and more particularly 0.8 hours to 1 hour. In a case where the time satisfies the above range, the diameter of the carbon nanotube structure can be appropriately controlled without breaking the carbon nanotube structure during the dispersion process, and a dispersion solution having a uniform composition can be prepared. The time during which bead milling is performed means the total time during which the bead mill is used, and, therefore, for example, if bead milling is performed several times, the performance time means the total time required to perform bead milling several times.

The above bead milling conditions are designed to disperse the bundle-type single-walled carbon nanotubes to an appropriate level, and specifically exclude the case in which the bundle-type single-walled carbon nanotubes are completely dispersed in single-stranded single-walled carbon nanotubes. That is, the above bead milling conditions are designed to form the carbon nanotube structure in which a plurality of single-walled carbon nanotube units are bonded together side by side in the conductive agent dispersion prepared by appropriately dispersing the bundle-type single-walled carbon nanotubes. This can be achieved only when the composition of the mixed solution and the dispersion process conditions (e.g., bead milling process) are strictly controlled.

A dispersion of carbon nanotube structures can be formed by the above procedure.

(2) Stage of arranging the carbon nanotube structure on the porous substrate

The carbon nanotube structure can be arranged on the porous substrate by applying the carbon nanotube structure dispersion onto the porous substrate and solidifying the applied carbon nanotube structure dispersion. The porous substrate may or may not include the inorganic coating layer described above, but it is preferable that it includes the inorganic coating layer in order to improve safety. A conventional method, such as a Mayer bar, a nozzle coater, a reverse roll coater, and an gravure coater, can be used to apply the carbon nanotube structure dispersion.

Secondary battery

A secondary battery according to another embodiment of the present invention may include an electrode and the separator of the embodiment described above.

The electrode may include a positive electrode and a negative electrode.

The electrode may include a layer of active electrode material. The electrode may further include a current collector, and in this case, the layer of active electrode material may be disposed on one or both surfaces of the current collector.

The current collector is not particularly limited as long as it has conductivity without causing adverse chemical changes in the battery, and, for example, copper, stainless steel, aluminum, nickel, titanium, alloys thereof can be used, these materials being surface treated with one of carbon, nickel, titanium, silver, or the like, or fired carbon.

The current collector may typically have a thickness of 3 |im to 500 |im, and microscopic irregularities may be formed on the collector surface to improve adhesion of the electrode active material. In addition, the electrode collector, for example, may be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam body, a nonwoven fabric body, and the like.

The electrode active material layer may include an electrode active material and a conductive agent.

The electrode active material may be a positive electrode active material or negative electrode active material commonly used in the art, but the types thereof are not particularly limited.

For example, as a positive electrode active material, a lithium oxide including lithium and at least one metal such as cobalt, manganese, nickel, or aluminum may be used. Specifically, the lithium oxide may include lithium-manganese based oxide (e.g., LiMnO<2>, LiMn<2>O, etc.), lithium-cobalt based oxide (e.g., LiCoO<2>, etc.), lithium-nickel based oxide (e.g., LiNiO<2>, etc.), lithium-nickel-manganese based oxide (e.g., LiNh_Y<1>MnY<1>O<2>(where 0<Y1<1), LiMn<2>-Z<1>Niz<1>O<4>(where 0<Z1<2), etc.), lithium-nickel-cobalt based oxide (e.g., LIN<1>-y<2>Coy<2>O<2>(where 0<Y2<1), lithium-manganese-cobalt based oxide (e.g., LiCo<1>-Y<3>MnY<3>O<2>(where 0<Y3<1), LiMn<2>-Z<2>CoZ<2>O<4>(where 0<Z2<2), etc.), lithium-nickel-cobalt-manganese-based oxide (e.g., Li(NiP<1>CoQ<1>MnR<1>)O<2>(where 0<P1<1, 0<Q1<1, 0<R1<1, and P1+Q1+R1=1) or Li(NiP<2>CoQ<2>MnR<2>)O<4>(where 0<P2<2, 0<Q2<2, 0<R2<2, and P2+Q2+R2=2), etc.), or lithium-nickel-cobalt-manganese-transition metal (M) oxide (e.g., Li(NiP<3>CoQ<3>MnR<3>M1S)O<2>(where M1 is selected from the group consisting of aluminum (Al), copper (Cu), iron (Fe), vanadium (V), chromium (Cr), titanium (Ti), zirconium (Zr), zinc (Zn), tantalum (Ta), niobium (Nb), magnesium (Mg), boron (B), tungsten (W), and molybdenum (Mo), and P3, Q3, R3, and S are the atomic fractions of each independent element, wherein 0<P3<1, 0<Q3<1, 0<R3<1, 0<S<1, and P3+Q3+R3+S=1), etc.), and any one thereof or a compound of two or more thereof may be included.

The negative electrode active material, for example, may include a carbonaceous material such as artificial graphite, natural graphite, graphitized carbon fibers, and amorphous carbon; a metal compound that can form an alloy with lithium such as silicon (Si), aluminum (Al), tin (Sn), lead (Pb), zinc (Zn), bismuth (Bi), indium (In), magnesium (Mg), gallium (Ga), cadmium (Cd), a Si alloy, a Sn alloy, or an Al alloy; a metal oxide that may be doped and undoped with lithium such as SiOv (0<v<2), SnO<2>, vanadium oxide, and lithium vanadium oxide; or a composite material that includes the metal compound and the carbonaceous material such as a Si-C composite material or an Sn-C composite material, and any one thereof or a mixture of two or more thereof may be used. In addition, a thin film of metallic lithium may be used as the negative electrode active material. In addition, both low-crystalline carbon and high-crystalline carbon can be used as carbon material.

The electrode active material may be included in an amount of 70% by weight to 99.5% by weight, for example, 80% by weight to 99% by weight based on the total weight of the electrode active material layer. When the amount of the electrode active material satisfies the above range, excellent energy density, electrode adhesion, and electrical conductivity can be achieved.

The conductive agent is not particularly limited as long as it has conductivity without causing adverse chemical changes in the battery, and conductive materials can be used, for example, graphite such as natural graphite and artificial graphite; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers or metal fibers; conductive tubes such as carbon nanotubes; metal powder such as fluorocarbon powder, aluminum powder, and nickel powder; conductive single-crystalline short fibers such as zinc oxide single-crystalline short fibers and potassium titanate single-crystalline short fibers; conductive metal oxide such as titanium oxide; or polyphenylene derivatives.

The electrode may further include a binder. The binder may include at least one selected from the group consisting of a polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylate, an ethylene-propylene-diene monomer (EPDM), a sulfonated EPDM, a styrene-butadiene rubber (SBR), a fluorinated rubber, polyacrylic acid, and a material having the hydrogen thereof substituted with lithium (Li), sodium (Na), or calcium (Ca), or may include various copolymers thereof.

The electrode is connected to a current-applying part.

Specifically, in a case where the secondary battery is a button cell, an external current is applied to the electrode in contact with a bottom can through the bottom can, and, in this case, the bottom can corresponds to the current applying portion. In a case where the secondary battery is a single cell (or a pouch-type or cylindrical-type battery having more stacks than the single cell) rather than a button cell, an external current is applied to the electrode through an electrode tab, and, in this case, the electrode tab corresponds to the current applying portion. In a case where the electrode is a positive electrode in the single cell, one end (an end region of a portion on which a positive electrode active material layer is not formed (uncoated portion)) of a positive electrode collector on the positive electrode may correspond to the current applying portion.

The separator may be disposed between the positive electrode and the negative electrode. The positive electrode includes a positive electrode collector and a positive electrode active material layer, and the negative electrode includes a negative electrode collector and a negative electrode active material layer. The positive electrode collector and the negative electrode collector include uncoated portions in which the positive electrode active material layer and the negative electrode active material layer are not arranged (do not overlap with the positive electrode active material layer), respectively. When explaining a case in which the positive electrode is in contact with the conductive layer of the separator (in some cases, the negative electrode, and not the positive electrode, may be in contact with the conductive layer of the separator), the conductive layer of the separator may be in contact with the uncoated portion of the positive electrode collector and the current applying portion (corresponding to a positive electrode tab). Therefore, the current flowing into the secondary battery through the current applying portion can be uniformly transmitted to the positive electrode active material layer through the conductive layer as well as the positive electrode collector.

Referring to Figure 7, when preparing a button cell, a separator 130 including a porous substrate 131 and a conductive layer 132 is disposed between a negative electrode 120 and a positive electrode 110 such that the conductive layer 132 of the separator 130 comes into contact with an upper surface of the positive electrode 110. Furthermore, a lower cap 111a is arranged to be in contact with a lower surface of the positive electrode 110, and a portion of the lower cap 111a comes into contact with the conductive layer 132 in an assembling method of the button cell. Therefore, a current applied through the lower cover 111a, which is a current applying part, not only spreads uniformly to the lower surface of the positive electrode 110 along the lower cover 111a, but can also spread uniformly to the upper surface of the positive electrode 110 along the conductive layer 132. Accordingly, the resistance of the battery can be reduced, and the input/output characteristics of the battery can be significantly improved.

A single cell will now be described. Referring to Fig. 8, the single cell includes a positive electrode 110 including a positive electrode collector 111 and a positive electrode active material layer 112, a negative electrode 120 including a negative electrode collector 121 and a negative electrode active material layer 122, and a separator 130 including a porous substrate 131 and a conductive layer 132, and an end portion of the positive electrode collector 111, in a state where the positive electrode active material layer 112 is not disposed on a surface thereof, protrudes far enough to constitute a positive electrode tab 111a, which is a current applying portion, and the positive electrode tabs 111a contact each other. In this case, a portion of the conductive layer 132 comes into contact (C) with a portion of the positive electrode tab 111a. Therefore, since a current applied through the positive electrode tab 111a not only flows to the positive electrode collector 111, but also flows to the conductive layer 132, the current can flow uniformly in the positive electrode active material layer 112. Accordingly, the resistance of the battery can be reduced, and the input/output characteristics of the battery can be significantly improved.

Figure 9 is a schematic diagram illustrating a positional relationship between a positive electrode collector 111, a positive electrode active material layer 112, a positive electrode tab 111a, and a conductive layer 132 of a separator in a jelly roll type battery in a state of an unwound battery before the jelly roll type battery is completed. The positive electrode collector 111 includes an uncoated portion that does not overlap with the positive electrode active material layer, and the uncoated portion and a portion of the positive electrode tab 111a come into contact with the conductive layer 132 (a region corresponding to C). Therefore, since a current applied through the positive electrode tab 111a not only flows to the positive electrode collector 111, but also flows to the conductive layer 132, the current can flow uniformly in the positive electrode active material layer 112. Consequently, the battery resistance can be reduced, and the input/output characteristics of the battery can be significantly improved.

10 and 11 , although similar to FIG. 9 , it can be understood that a plurality of positive electrode tabs 111a are present, and a plurality of positive electrode active material layers 112 are also present in separate regions. In this case, since a region of the positive electrode tab 111a and a region of the uncoated portion, which are in contact with the conductive layer, are increased, a current applied across the plurality of positive electrode tabs 111a flows more evenly into the conductive layer 132, and thus, the current can flow more evenly in the positive electrode active material layer 112. Accordingly, the resistance of the battery can be further reduced, and the input/output characteristics of the battery can be significantly improved.

In Figures 7 to 11 described above, the positions of the positive electrode and the negative electrode can be exchanged with each other.

The secondary battery may further include an electrolyte. The electrolyte may include an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, or a melt-type inorganic electrolyte that can be used in preparing the lithium secondary battery, but the present invention is not limited thereto.

Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt.

As the non-aqueous organic solvent, for example, an aprotic solvent such as N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, y-butyrolactone, 1,2-dimethoxyethane, tetrahydroxy franc, 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphate triester, trimethoxymethane, a dioxolane derivative, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, a propylene carbonate derivative, a tetrahydrofuran derivative, ether, methyl propionate, and ethyl propionate may be used.

Particularly, among the carbonate-based organic solvents, since ethylene carbonate and propylene carbonate, as cyclic carbonates, dissociate lithium salts well due to their high permittivity as highly viscous organic solvents, cyclic carbonates can be preferably used. Since an electrolyte with high electrical conductivity can be prepared when the above cyclic carbonate is mixed with linear carbonates of low viscosity and low permittivity, such as dimethyl carbonate and diethyl carbonate, in an appropriate ratio and used, cyclic carbonates can be more preferably used.

As the metal salt, a lithium salt may be used, and the lithium salt is a material that is readily soluble in the non-aqueous organic solvent, wherein, for example, as the anion of the lithium salt, at least one selected from the group consisting of F-, Cl-, I", NO<3>-, N(CN)<2>", BF<4>", CO<4>", PF6", (CF3^PF4-, (CF3^PF3-, (CF3^PF2-, (CF<3>)<5>PF-, (CF3)aP-, CF<3>SO<3>- CF<3>CF<2>SO<3>-, (CF<3>SO<2>)<2>N-, (FSO<2>)<2>N-, CF<3>CF<2>(CF<3>)<2>CO-, (CF<3>SO<2>)<2>CH-, (SF<5>)<3>C-, may be used. (CF3SO2)3C-, CF3(CF2)7SO3-, CF<3>CO<2>-, CH<3>CO<2>-, SCN-, and (CF3CF2SO2)2N-.

In order to improve the life characteristics of the battery, suppress the reduction in battery capacity, and improve the discharge capability of the battery, in addition to the electrolyte components, at least one additive may be further added to the electrolyte, for example, a haloalkylene carbonate-based compound such as difluoroethylene carbonate, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphoric triamide, a nitrobenzene derivative, sulfur, a quinoneimine dye, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, an ammonium salt, pyrrole, 2-methoxyethanol, or aluminum trichloride.

According to another embodiment of the present invention, a battery module including the secondary battery as a unit cell and a battery pack including the battery module are provided. Since the battery module and the battery pack include the secondary battery having high capacity, high rate capability, and high cycle characteristics, the battery module and the battery pack can be used as a power source for a medium-sized and large-sized device selected from the group consisting of an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, and an energy storage system.

Preferred examples are presented hereinafter in order to aid in a better understanding of the present invention, but the following examples are presented merely to exemplify the present invention, it will be apparent to those skilled in the art that various modifications and alterations are possible within the scope and technical spirit of the present invention, and such modifications and alterations are within the scope of the claims included herein.

Preparation Example 1: Preparation of carbon nanotube structure dispersion

0.4 part by weight of bundle-type carbon nanotubes (specific surface area of 650 m2/g) composed of single-walled carbon nanotube units having an average diameter of 1.5 nm and an average length of 5 |im or more and 0.6 part by weight of carboxymethylcellulose (weight-average molecular weight: 400,000 g/mol, degree of substitution: 1.0) were mixed in 99.0 parts by weight of water as a dispersion medium to prepare a mixture so that the solid content was 1.0% by weight.

The bundle-type single-walled carbon nanotubes were dispersed in the solvent by stirring the mixture using a bead milling method, thereby preparing a carbon nanotube structure dispersion. In this case, the beads had a diameter of 1 mm, the rotation speed of the stirring vessel containing the beads was 3,000 rpm, and stirring was carried out for 60 minutes. The carbon nanotube structure dispersion included a carbon nanotube structure in a form in which 2 to 5,000 single-walled carbon nanotube units were linked side by side (see Figure 5). In the carbon nanotube structure dispersion, the amount of the carbon nanotube structure was 0.4% by weight, and the amount of carboxymethylcellulose was 0.6% by weight.

Preparation Example 2: Preparation of Dispersion of Single-Walled Carbon Nanotube Units 0.2 parts by weight of bundle-type carbon nanotubes (specific surface area of 650 m2/g) composed of single-walled carbon nanotube units having an average diameter of 1.5 nm and an average length of 5 |im or more and 1.2 parts by weight of carboxymethylcellulose (weight-average molecular weight: 100,000 g/mol, degree of substitution: 1.0) were mixed in 98.6 parts by weight of water, as a dispersion medium, to prepare a mixture so that the solid content was 1.4% by weight.

The bundle-type single-walled carbon nanotubes were dispersed in the solvent by stirring the mixture using a bead milling method, thereby preparing a conductive agent dispersion. In this case, the beads had a diameter of 1 mm, the rotation speed of the stirring vessel containing the beads was 3,000 rpm, and stirring for 60 minutes under the above conditions was set as one cycle, and a total of 4 cycles were performed (natural cooling was performed for 60 minutes between each cycle). Consequently, a dispersion of single-walled carbon nanotube units was prepared. In the dispersion, since the bundle-type single-walled carbon nanotubes were completely dispersed, the single-walled carbon nanotube unit existed only as a single-chain unit, but the carbon nanotube structure described above was not detected (see Figure 6). Furthermore, in the dispersion of single-walled carbon nanotube units, the amount of the single-walled carbon nanotube unit was 0.2% by weight, and the amount of the carboxymethylcellulose was 1.2% by weight.

Examples and comparative examples

Example 1: Preparation of separator and secondary battery

(1) Separator preparation

Acetone and a PVDF-HFP binder (weight average molecular weight: 300,000 g/mol) were mixed to prepare a polymer solution (solid content concentration of 5% by weight). AhO<3> (Nippon Light Metal Company, Ltd., LS235) was added to the polymer solution in an amount of 20% by weight based on the total amount of the polymer solution and then dispersed by a ball milling method to prepare a slurry for an inorganic porous coating layer. The slurry was coated onto a porous substrate (Toray Industries, Inc. B12PA1, thickness of 12 |im) by a dip coating method, and phase separation was induced under humidified conditions at a relative humidity (RH) of about 40%. A separator including an inorganic coating layer was prepared by such a method. The carbon nanotube structure dispersion of Preparation Example 1 was surface coated on the separator prepared by the above method to have a thickness of 500 nm based on a cross section by a doctor blade coating method, and dried in an oven at 120 °C for 1 minute to prepare a separator.

Li[Ni<0>,<6>Mn<0>,<2>Co<0>,<2>]O<2> was used as the positive electrode active material. The positive electrode active material, carbon black as a conductive agent, and polyvinylidene fluoride (PVdF) as a binder were mixed in an N-methyl-2-pyrrolidone solvent in a weight ratio of 94:4:2 to prepare a positive electrode suspension.

The prepared suspension was applied to a 15 μm thick positive electrode collector (Al) at a loading rate of 5 mAh/cm2 and dried. The circulating air temperature was 110 °C. The positive electrode collector was then laminated and dried in a vacuum oven at 130 °C for 2 hours to form a positive electrode active material layer.

Artificial graphite as a negative electrode active material, carbon black as a negative electrode conductive agent, a styrene-butadiene rubber (SBR) as a negative electrode binder, and carboxymethyl cellulose (CMC) were mixed in distilled water in a weight ratio of 96.1:0.5:2.3:1.1 to prepare a negative electrode slurry. A 20 μm-thick negative electrode collector (Cu) was coated with the prepared slurry so that the charging amount was 6 mAh/cm2 and dried. Thereafter, the negative electrode collector on which the negative electrode slurry was disposed was laminated by a roller lamination method so that the total thickness of the negative electrode slurry and the negative electrode slurry was 80 μm. Thereafter, the negative electrode slurry and the negative electrode slurry were dried at 110 °C for 6 hours to prepare a negative electrode.

After that, after preparing a single cell by combining the positive electrode and the negative electrode prepared above with each of the separators of the examples and comparative examples arranged therebetween, an electrolyte solution (ethylene carbonate (EC)/ethyl methyl carbonate (EMC) = 1/2 (volume ratio), lithium hexafluorophosphate (1 M LiPF6)) was injected into the single cell to finally prepare a lithium secondary battery. A portion of a conductive layer on the separator and a portion of a positive electrode tab, which was one end of the positive electrode collector of the positive electrode, were in contact with each other. Example 2: Preparation of separator and secondary battery

A separator and a battery were prepared in the same manner as in Example 1, except that the thickness of a conductive layer was 250 nm.

Example 3: Preparation of separator and secondary battery

A separator and a battery were prepared in the same manner as in Example 1, except that the charge amount of a positive electrode was 2.5 mAh/cm2

Example 4: Preparation of separator and secondary battery

A separator and a battery were prepared in the same manner as in Example 1, except that the thickness of a conductive layer was 250 nm and the charge amount of a positive electrode was 2.5 mAh/cm2.

Comparative example 1: Preparation of separator and secondary battery

A separator and battery were prepared in the same manner as in Example 1, except that no conductive layer was formed.

Comparative example 2: Preparation of separator and secondary battery

A separator and battery were prepared in the same manner as in Example 3, except that no conductive layer was formed.

Comparative example 3: Preparation of separator and secondary battery

A separator and a battery were prepared in the same manner as in Preparation Example 1, except that the dispersion of single-walled carbon nanotube units of Preparation Example 2 was used instead of the dispersion of carbon nanotube structures of Preparation Example 1.

Comparative example 4: Preparation of separator and secondary battery

A separator and a battery were prepared in the same manner as in Preparation Example 2, except that the dispersion of single-walled carbon nanotube units of Preparation Example 2 was used instead of the dispersion of carbon nanotube structures of Preparation Example 1.

Comparative example 5: Preparation of separator and secondary battery

A separator and a battery were prepared in the same manner as in Example 3, except that the dispersion of single-walled carbon nanotube units of Preparation Example 2 was used instead of the dispersion of carbon nanotube structures of Preparation Example 1.

Comparative example 6: Preparation of separator and secondary battery

A separator and a battery were prepared in the same manner as in Example 4, except that the dispersion of single-walled carbon nanotube units from Preparation Example 2 was used instead of the dispersion of carbon nanotube structures from Preparation Example 1.

[Table 1]

In Examples 1 to 4, the average diameter of the carbon nanotube structure was 100 nm, and the average length was 15.6 μm. The average diameter and average length corresponded to an average of the diameters and lengths of the first 100 carbon nanotube structures with larger diameters (or lengths) and the last 100 carbon nanotube structures with smaller diameters (or lengths) when the prepared negative electrode was observed by TEM. In Comparative Examples 2 and 4, the average diameter of the single-walled carbon nanotube unit was 1.6 nm, and the average length was 1.8 μm. The average diameter and average length corresponded to an average value of diameters and lengths of the first 100 SWC units with a larger diameter (or length) and the last 100 SWC units with a smaller diameter (or length) when the prepared negative electrode was observed by TEM.

Experimental example 1: Observation of the separators

The separators of Example 1 and Comparative Example 2 were observed using a SEM.

Figure 1 is an SEM image of the separator of Comparative Example 1, Figure 2 is an SEM image of the separator of Comparative Example 1, Figure 3 is an SEM image of the separator of Comparative Example 2, and Figure 4 is an SEM image of the separator of Comparative Example 3.

Referring to Figures 2 and 3, thick rope-shaped carbon nanotube structures (a form in which 2 to 5,000 single-walled carbon nanotube units are linked together side by side) were arranged on a surface of an inorganic coating layer to form a network, and the carbon nanotube structures constitute a uniformly tangled network structure without blocking the pore structure on the surface of the inorganic coating layer. In addition, carboxymethylcellulose was arranged on the carbon nanotube structure. In contrast, referring to Figure 4, no thick, long carbon nanotube structure was observed, and a thin, short single-walled carbon nanotube unit was present as a single-stranded unit. Therefore, if the same amount was used, the number of single-walled carbon nanotube units relative to the number of carbon nanotube structures would increase exponentially, and in this case, since the single-walled carbon nanotube units were densely packed together to such an extent that the surface pore structure was completely blocked, the single-walled carbon nanotube units not only arranged in the pores, but also arranged while coating the inorganic layer to form a kind of surface layer without any gaps.

Experimental example 2: Evaluation of the air permeability of the separator

Air permeability was evaluated for the separators prepared in Examples 1 to 4 and Comparative Examples 1 to 6 as follows.

The time (s) required for 100 ml of air to penetrate through the separator at a constant pressure (0.05 MPa) was measured with an air permeability tester (manufacturer: Asahi Seiko, product name:

EG01-55-1MR). An average was calculated by measuring a total of 3 points including 1 point each on the left/center/right side of each separator, and the results are presented in Table 2. If the air permeability is 500 s/100 cm3 or more, it may cause a decrease in battery output and a degradation in cycling characteristics.

Experimental example 3: Evaluation of the surface resistance of the separator's conductive layer

The surface resistance of the conductive layer for the separators prepared in Examples 1 to 4 and Comparative Examples 1 to 6 was evaluated as follows.

The surface resistance of the separator's conductive layer was measured using a surface resistance meter (manufacturer: Mitsubishi, product name: MCP-T610) equipped with a 4-pin probe. An average was calculated by measuring 1 point each on the left/center/right side (a total of 3 points) of each separator, and the results are presented in Table 2.

Experimental Example 4: Evaluation of discharge capacity according to the C rate

The output characteristics for the batteries of Examples 1 to 4 and Comparative Examples 1 to 6 were evaluated as follows.

A charge C rate was set at 0.2 C, and then the ratio of discharge capacity at 2.0 C (%) to discharge capacity at 0.2 C was measured for each lithium secondary battery while increasing the discharge C rate from 0.2 C to 2.0 C, and the results are presented in Table 2.

[Table 2]

Claims (12)

i. A separator comprising a porous substrate and a conductive layer disposed on the porous substrate, wherein the conductive layer comprises a carbon nanotube structure in which a plurality of single-walled carbon nanotube units are bonded together side by side, and wherein the carbon nanotube structure has an average diameter of 2 nm to 500 nm. 2. Separator according to claim 1, wherein the carbon nanotube structures are interconnected in the conductive layer to form a network structure. 3. Separator according to claim 1, wherein the carbon nanotube structure has an average length of 1 |im to 500 |im. 4. Separator according to claim 1, wherein the conductive layer has a thickness of 10 nm to 2,000 nm. 5. Separator according to claim 1, wherein the conductive layer has a surface resistance of 5*10-1 Q/aa5*104 Q/a. 6. Separator according to claim 1, wherein the conductive layer further comprises an additive that covers at least a portion of a surface of the carbon nanotube structure. 7. Separator according to claim 6, wherein the additive comprises a carboxymethylcellulose. 8. Separator according to claim 1, wherein the air permeability is in a range of 50 s/100 cm3 to 500 s/100 cm3. 9. Separator according to claim 1, further comprising an inorganic coating layer comprising inorganic particles between the porous substrate and the conductive layer. 10. Separator according to claim 1, wherein the conductive layer is arranged on a surface of the porous substrate. 11. Secondary battery comprising an electrode and the separator according to claim 1. 12. Secondary battery according to claim 11, wherein the electrode comprises a positive electrode and a negative electrode, in which the separator is arranged between the positive electrode and the negative electrode, wherein the positive electrode comprises a positive electrode collector and a layer of positive electrode active material, wherein the positive electrode collector comprises an uncoated portion that does not overlap the positive electrode active material layer, wherein the uncoated portion comprises a current applying portion corresponding to an end region of the uncoated portion, and in which the conductive layer is in contact with the current-applying part.